Testing Suite (and setup/teardown) functionality

I've got the suite functionality working to my satisfaction.  I've updated the documentation (and tried to mostly match the already-existing documentation style) to explain what the suite package provides and how it might be used.  The suite_test.go file contains a full, working test suite, which can be (and is, currently) used both as an example and for testing the suite logic to make sure it's running as intended.

Some of the links in README.md that I've added are currently dead, but that's because they're pointing to the master branch (which won't contain those links until this pull request is merged) and testify's documentation at go.pkgdoc.org (which has a similar issue) - I've done my best to make sure the links will be correct once this pull request is merged and the documentation is updated on go.pkgdoc.org.